Method of using low viscosity-low volatility mineral oil and wax in an insulated electric cable



Nov. 24, 1959 A. KING ETAL 2,914,430

METHOD OF usmc LOW VISCOSITY-LOW VOLATILITY MINERAL 011. AND WAX m m INSULATED ELECTRIC CABLE Filed Feb. 1a. 195:

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paper screen 5 Jute serwhg Jute Sen [I79 7 6 Stee/ Wm? amour/n9 A ffok/VE) United States Patent METHOD OF USING LOW VISCOSITY-LOW VOLA- 1 TILITY MINERAL OIL AND WAX IN AN IN- SULATED ELECTRIC CABLE Albert King, Bexley Heath, and Kenneth John Collins,

Barnehurst, England, assign'ors to British Insulated Callenders Cables Limited, London, England, a British company Application February 18, 1953, Serial No. 337,626 Claims priority, application Great Britain March 3, 1952 7 Claims. (Cl. 117-231) This invention relates to insulated electric cables having .a dielectric of impregnated fibrous insulating material, generally paper, and in particular to fully impregnated non-draining cables of this kind and to impregnating compounds therefor.- By stating that the cable is fully impregnated it is intended to imply that the cable is as full of impregnant as is possible, the paper or other fibrous material being impregnated and the spaces between the turns and layers thereof being filled with compound, the object being to ensure that the possibility of the existence of voids within the sheath which surrounds the dielectric is reduced to a minimum, and by a non-draining cable is meant a cable such that when a vertical length of 30 feet or more of the cable, with open ends, is heated to its maximum rated conductor working temperature for 7 days or more the amount of impregnating compound drained from the cable does not exceed 0.1% by volume of the cubical capacity of the cable sheath.

In the specification of our prior Patent No. 2,586,345 there is described and claimed a fully impregnated nondraining cable in which the cable impregnant is a normally plastic solid compound consisting of micro-crystalline petroleum wax having a melting point of not less than 80 C. and of cable impregnating mineral oil of one of a group of plasticizers consisting of polyisobutene (polyisobutylene) having an average molecular weight of 20,000 to 100,000 and rosin or a mixture of such oil with one or both of such plasticizers, the proportion of micro-crystalline wax in the compound being not less than 55% by weight. An improved form of such cable is described and claimed in our co-pending application Serial No. 274,596 in which instead of the micro-crystah line petroleum wax compound of the grade described in the specification of Patent 2,586,345 there is used microcrystalline petroleum wax having a melting point of about 88 C. and needle penetration values of about 5 at 25 C. and about 35 at 70 C. or a blend of such wax with micro-crystalline petroleum waxes having higher penetration values at high temperatures, having a melting point of 8590 C. and needle penetration values varying from about 5 and 6 at 25 C. to between 35 and 50 at 70 C., the proportion of micro-crystalline wax in the compoundbeing not less than about 48% by weight when wax having a needle penetration value of 35 at 70 C. is used and not less than about 50% by weight when the specified blend of wax is used.

By the term micro-crystalline petroleum wax used above and hereinafter is meant wax of the petroleum group or petroleum ceresin group of petroleum waxes (as defined and classified in the Journal of the Institute of Petroleum, vol. 29, No. 240, 1943, pp. 361-3), which has a relatively small crystal size compared with that of waxes of the parafiin wax group. Such wax iscurrently known as micro-crystalline wax or amorphous wax. The needle penetration values referred to above and those referred. hereinafter in thespecification and the claims Patented Nov. 24, 1959 'ice forming part thereof are needle penetration values as determined in accordance with Method I.P.49/46 of the Institute of Petroleum.

The cable impregnating mineral oils referred to in the aforesaid patent specifications are those which, at the date of the aforesaid patent, were generally known amongst cable makers to be suitable for the impregnation of paper insulated electric cables. They are discussed, for instance, in the symposium of papers on insulating oils in the Journal of the Institution of Electrical Engineers, part II, pages 364, vol. 90, 1943. They vary in viscosity from oils having a viscosity of about 11,000 centistokes or higher at 0 C. to the then so-called Hollow Core Cable Oil having a viscosity of about 140 centistokes at 0 C.

We have now discovered that improved micro-crystalline wax impregnating compounds are obtained by using for the production of such compounds mineral oils of very low viscosity and of low volatility. Here and hereafter in this specification and in the claims forming part thereof, by an oil of very low viscosity we mean one having a viscosity of less than centistokes at 0 C. and by an oil of low volatility we mean an oil that shows a loss of less than 2% when heated for 5 hours in air at approximately C. as specified in Method I.P. 46/51 of the Institute of Petroleum. Accordingly the present invention comprises a cable impregnating compound which is a normally plastic solid consisting essentially of micro-crystalline petroleum wax having a melting point of not less than 80 C. and mineral oil of very low viscosity and of low volatility. The invention also comprises a fully impregnated non-drain-. ing cable in which the impregnant within the paper (or other fibrous insulating material) and in the interstices between the turns and layers thereof is a normally plastic solid consisting essentially of micro-crystalline petroleum wax having a melting point of not less than 80 C. and mineral oil of very low viscosity and low volatility. The impregnating compound in addition to its essential components may include one or more plasticizers such as, for example, polyisobutene, rosin, polyethylene and butyl rubber.

Such very low viscosity oils have been found to be bet-. ter retained by micro-crystalline petroleum wax than oils of the grades mentioned in the specifications of the aforesaid prior patent and co-pending patent application, with the result that cables impregnated with our improved compounds have even less tendency to drain at temperatures approaching the melting point of the compound than have cables made in accordance with the aforesaid patent or co-pending application. It may appear surprising that, other conditions being equal, better nondraining cable can be obtained when using a compound containing oil of very low viscosity than when using oil of substantially higher viscosity but we believe it to be due to the smaller molecules of the very low viscosity oil becoming absorbed more completely into the crystal structure of the wax.

The proportions of micro-crystalline wax and very low viscosity oil and other ingredients, if any, that may be used will depend upon the grade of micro-crystalline wax used and upon the viscosity of the oil used and also upon the conditions under which the cable is required to operate. Where micro-crystalline Wax having a melting point of about 88 C., and needle penetration values of about 5 at 25 C. and about 35 at 70 C. is used, the wax content may be as low as about 35% by weight. Where micro-crystalline wax having a melting point of 80 C. or higher and needle penetration values of about 6 at 25 C. and from about 75 to at 70 C. is used 1 namely, about 45% by weight. Where a blend of such micro-crystalline waxes, or an equivalent micro-crystalline petroleum wax, having a melting point of not less than 80 C. and needle penetration values of between and 6' at C. and from about to 75 at 70 C. is used, the minimum wax content will lie between the two values quoted and will preferably be about The very low viscosity oil content should not be less than 30% and is preferably at least 40% by weight of the compound.

The degree of the improvement obtained by the present invention will be apparent from a comparison ofthe Absorption Indexes of compounds made in accordance with the present invention with those of compounds of the kind disclosed in the specification of our co-pending application No. 274,596.

It is explained that we have found that cables impregmated with micro-crystalline waxes and impregnating compounds containing them are not necessarily free from draining tendencies merely because the impregnants have melting points higher than the temperatures at which the cables are tested or operated. However in the course of our experiments in connection with non-draining cables we have developed an empirical test (which we have termed the Absorption Test) which we have found to give a reliable indication of the non-draining characteristics of impregnated paper insulated cables under operating conditions. In this test the absorptive propcity of the paper (or other fibrous cable dielectric material) is matched against the ability of micro-crystalline wax impregnating compounds to retain their oil and plasticiser contents. The test comprises placing a cast sample of the compound of standard dimensions in contact with a sheet of paper of the grade to be used in the manufacture of the cable dielectric and maintaining the assembly in a horizontal position at a given raised temperature for a given period of time during which oil from the sample is absorbed by the paper to an extent dependent upon the characteristics of the compound and upon the chosen conditions of test, and observing the extent of the absorption. The perimeter of the oily patch of paper produced is substantially the circumference of a circle. The difference between this perimeter and the circumference of the sample under test, measured in centimetres, we term the absorption index of the sample.

With one exception the absorption index figures quoted hereinafter have been determined under the following test conditions:

(1) Samples were prepared by casting the molten compound in a watch glass having a diameter of 3.8 cms., a circumference of approximately 12 cms. and a depth of concavity of 0.4 cms.

(2) The flat surface of the cast sample was placed over the centre of a rigidly held six inch square of cable insulating paper.

(3) The cable insulating paper was of sulphate wood type, and had a thickness of 5.25 mils (0.13 mm), an apparent density relative humidity and 20 C.) of 0.78 and a Gurley porosity of 150 seconds per 100 cc.

(4) The temperature and duration of the test were respectively, 170 F. (76.7 C.) and 16 hours.

We have found that a paper insulated cable impregnated with a micro-crystalline wax impregnating compound having a melting point of not less than C. and under the aforesaid test conditions an absorption index of 15 or less, will operate as a non-draining cable at conductor temperatures up to 70 C. and cable impregnated with a micro-crystalline wax impregnating compound having a melting point of not less than 82 C. and an absorption index of about 10 or less will operate as non-draining cable with conductor temperatures up to C. The maximum permissible absorption index for other operating temperatures and papers or other dielect'ric materials can readily be determined experimentally.

Table I gives properties of examples of micro=crystal= line wax impregnating compounds, suitable for use in the manufacture of fully impregnated non-draining paper insulated cable. In all examples the grade of microcrystalline petroleum Wax used was that sold by the British Sun Oil Company Limited under the trade name Sunoco 985 Yellow Micro-crystalline Wax and having a melting point of about 88 C. and needle penetration values of about 5 at 25 C. and about 35 at 70 C. and hereinafter referred to as Sunoco 985. The oil used in Examples 1-3 inclusive and designated O.F.C. oil was a Hollow Core Cable Oil (oil-filled cable oil) having a viscosity of 140 centistokes at 0 C., 35 centistokes at 20 C. and 7.3 centistokes at 60 C., that is to say, it was similar to that specified in Examples 1-4 given in the complete specification of our co-pending application No. 274,596. The oil used in Examples 4-7 inclusive and designated Oil Awas a very low viscosity mineral oil having a viscosity of 80 centistokes at 0 C., 26 centistokes at 20 C. and 6.2 centistokes at 60 C. The oil used in Examples 8-12 inclusive and designated Oil B was a very low viscosity mineral oil having a viscosity of 35 centistokes at 0 C., 13 centistokes at 20 C. and 3.5 centistokes at 60 C. Thus it will be seen that Examples 1 and 2 are examples of microcrystalline petroleum wax compounds for the manufacture of cable according to application 274,596 and Examples 4, 5 and 6 and 812 inclusive are examples of micro-crystalline petroleum wax compounds for the manufacture of cable according to the present application.

Table 1 Compound, Parts v by weight Melting Cone Absorp- Example N0. Point, Penetion tration, Index Wax O.F.C. 25 0.

Oil

Oil A Oil B 1 Taken at 70 C.

v The cone penetration figures were determined by Method I.P.50/ 48 of the Institute of Petroleum.

From the Absorption Index fingers given in Table I it will be seen that cable impregnated with microcrystalline Wax-oil compounds containing very low viscosity oils are less likely to drain than cables impregnated with micro-crystalline wax compounds having the same wax content but containing oil filled cable oil or oil of higher viscosity. From Table I it will be seen that the minimum percentage of wax required to obtain compound having an absorption index of 15 or less is lowered from 48 to 44 by using Oil A in place of O.F.C. Oil and from 48 to about 38 by using Oil B of still lower viscosity in place of O.F.C. Oil. Thus where microcrystalline wax having a melting point of about 88 C. and needle penetration values of about 5 at 25 C. and about 35 at 70 C. is used, satisfactory compounds are obtained with a wax content of from about 38 to 60% by weight and an oil content of from about 62 to 40% byweight, providing the viscosity of the oil used is such as will result in the compound having an absorption in:

dex of 15 or less. When using Oil B, the minimum percentage of wax may be as low as 35, for the compound of Example 12 is suitable for the manufacture of nondraining cable operating at conductor temperatures of about 65 C. The compound of Example 10 could be used for the manufacture of cable operating at conductor temperatures up to 70 C. with a substantial margin of safety from drainage, and that of Example 4 could be used for the production of a non-draining cable operating at conductor temperatures of about 80 C. By the present invention therefore we provide an insulated electric cable having a dielectric of impregnated fibrous material in which the impregnant within the fibrous material and in the interstices between the turns and layers thereof is a normally plastic solid having an Absorption Index not greater than 15 and consisting of microcrystalline petroleum wax having a melting point of not less than 80 C. and of very low viscosity mineral oil, with or without one or more suitable plasticisers, for example polyisobutene, rosin, polyethylene and butyl rubher.

The penetration/temperature curves of our improved non-draining cable compounds containing mineral oil of very low viscosity are not steep at temperatures below 25 C. and at 25 C. they have high penetrations. Consequently non-draining cable made in accordance with the invention has very satisfactory bending characteristics at normal atmospheric temperatures, whilst a supertension non-draining cable impregnated wtih the compound of Example 12 will withstand severe bending tests at about C. without any paper rupture.

A further advantage of using compounds comprising a high proportion of very low viscosity oil is that such compounds have lower coefficients of thermal expansion over the range 20-l00 C. than have compounds of approximately the same Absorption Index made from O.F.C. oil or oil of higher viscosity, thus mitigating to an important extent the effects of void formation within the dielectric set up as the compound cools from its solidification temperature to normal temperature.

The accompanying drawing which is not to scale shows in cross-section an example of a three-core paper insulated mine shaft cable made in accordance with the invention. Each core comprises a sector-shaped, stranded copper Wire conductor 1 on which is built up a body of insulation consisting of helically lapped paper tapes 2. Surrounding each insulated conductor is a conductive screen 3. This screen, of which the thickness has been exaggerated to show it more clearly, is of metallised paper. Enclosing the three laid up screened cores is a lead sheath 4. In the interstices between the cores and those of the laminated paper body on each conductor and impregnating the paper of which the body is built up is a normally plastic solid comprising micro-crystalline petroleum wax having a melting point of not less than 80 C. and mineral oil of very low viscosity and low volatility. Over the lead sheath there is a protective layer 5 of impregnated jute on which is laid a steel Wire annouring 6 which, in turn, is covered by a second protective layer 7 of impregnated jute.

What we claim as our invention is:

1. In the manufacture of electric cable having a dielectric of impregnated fibrous insulating material, the method which comprises impregnating the insulating material with an impregnating compound which is a normally plastic solid and which comprises as essential ingredients micro-crystalline petroleum wax having a melting point of not less than 80 C. and not less than 30% by weight of mineral oil having a viscosity of less than 100 centistokes at 0 C. and showing a loss of less than 2% when heated for 5 hours in air at approximately 110 C. as specified in Method LP. 46/51 of the Institute of Petrole- 2. In the manufacture of electric cable having a dielectric of impregnated fibrous insulating material, the

method which comprises impregnating the insulating material with an impregnating compound which is a normally plastic solid and which comprises as essential ingredients at least about 35% by weight of micro-crystalline petroleum wax having a melting point of about 88 C. and needle penetration values of about 5 at 25 C. and about 35 at 70 C. and at least 30% by weight of mineral oil having a viscosity of less than 100 centistokes at '0 C. and showing a loss of less than 2% when heated for 5 hours in air at approximately 110 C. as specified in Method LP. 46/51 of the Institute of Petroleum.

3. In the manufacture of electric cable having a dielectric of impregnated fibrous insulating material, the method which comprises impregnating the insulating material with an impregnating compound which is a normally plastic solid and which comprises as essential ingredients at least about 45% by weight of micro-crystalline petroleum wax having a melting point of not less than C. and needle penetration values of about 6 at 25 C. and from about '75 to 150 at 70 C. and at least 30% by Weight of mineral oil having a viscosity of less than centistokes at 0 C. and showing a loss of less than 2% when heated for 5 hours in air at approximately C. as specified in Method LP. 46/51 of the Institute of Petroleum.

4. In the manufacture of electric cable having a dielec trio of impregnated fibrous insulating material, the method which comprises impregnating the insulating material with an impregnating compound which is a normally plastic solid and which comprises as essential ingredients not less than about 40% by weight of micro-crystalline petroleum wax having a melting point of not less than 80 C. and needle penetration values of between 5 and 6 at 25 C. and from about 35 to 75 at 70 C. and not less than 30% by weight of mineral oil having a viscosity of less than 100 centistokes at 0 C. and showing a loss of less than 2% when heated for 5 hours in air at approximately 110 C. as specified in Method LP. 46/51 of the Institute of Petroleum.

5. In the manufacture of electric cable having a dielectric of impregnated fibrous insulating material, the method which comprises impregnating the insulating material with an impregnating compound which is a normally plastic solid and which comprises as essential ingredients from about 35 to 60% by weight of micro-crystalline petroleum wax having a melting point of about 88 C. and needle penetration values of about 5 at 25 C. and about 35 at 70 C. and from about 65 to 40% by weight of mineral oil having a viscosity of less than 100 centistokes at 0 C. and showing a loss of less than 2% when heated for 5 hours in air at approximately 110 C. as specified in Method I.P. 46/51 of the Institute of Petroleum.

6. In the manufacture of electric cable having a dielectric of impregnated fibrous insulating material, the method which comprises impregnating the insulating material with an impregnating compound which is a normally plastic solid and which comprises as essential ingredients about 44 to 60% by weight of micro-crystalline petroleum wax having a melting point of about 88 C.

and needle penetration values of about 5 at 25 C. and

about 35 at 70 C. and about 56 to 40% by weight of mineral oil having a viscosity of about 80 centistokes at 60 C. and showing a loss of less than 2% when heated for 5 hours in air at approximately 110 C. as specified in Method I.P. 46/51 of the Institute of Petroleum.

7. In the manufacture of electric cable having a dielectric of impregnated fibrous insulating material, the method which comprises impregnating the insulating material with an impregnating compound which is a normally plastic solid and which comprises as essential ingredients about 38 to 60% by weight of micro-crystalline petroleum wax having a melting point of about 88 C. and needle penetration values of about 5 at 25 C. and

References Cited in the file of this patent UNITED STATES PATENTS 2,120,306 Zender June 14, 1938 8 Soday "Feb. -11, 1947 Maier et a1. May 11, 1948 Robinson June 27, 1950 Pabst et al. July 24, 1951 King Feb. 19, 1952 McCue et a1. Apr. 29, 1952 OTHER REFERENCES Publication, Modern Packaging, volumev 17, January 1944, Number 5, Petroleum Waxes, pages 48, 49 and 

1. IN THE MANUFACTURE OF ELECTRIC CABLE HAVING A DIELECTRIC OF IMPREGNATED FIBROUS INSULATING MATERIAL, THE METHOD WHICH COMPRISES IMPREGNATING THE INSULATING MATERIAL WITH AN IMPREGNATING COMPOUND WHICH IS A NORMALLY PLASTIC SOLID AND WHICH COMPRISES AS ESSENTIAL INGREDIENTS MICRO-CRYSTALLING PETROLEUM WAX HAVING A MELTING POINT OF NOT LESS THAN 80*C, AND NOT LESS THAN 30% BY WEIGHT OF MINERAL OIL HAVING A VISCOSITY OF LESS THAN 100 CENTISTROKES AT 0*C, AND SHOWING A LOSS OF LESS THAN 2% WHEN HEATED FOR 5 HOURS IN AIR AT APPROXIMATELY 110 * C. AS SPECIFIED IN METHOD I.P. 46/51 OF THE INSTITUTE OF PETROLEUM. 